Numerous appliances, notably portable telephones, use an antenna employing planar microstrip technology for their flexible and easily integratable structure.
However, this antenna must meet certain criteria such as have a wide bandwidth, large gain, reduced proportions and be low cost in order to integrate it into these appliances. These criteria often cannot be complied with at the same time, notably in respect of bandwidth, good efficiency (large gain) and reduced proportions. In particular, to have good efficiency, the bandwidth of this antenna is generally low, of the order of 5%.
Several techniques based on modifying the geometry of the antenna have been proposed for widening the bandwidth to the detriment of the proportions of the antenna. Other techniques rely on the use of lossy dielectric substrates, the insertion of slots on the radiating element, the use of the near context, and the use of materials having high-impedance surfaces.
An example of such an antenna is given by the article “Stacked H-shaped microstrip patch antenna”, published in 2004 in Antennas and Propagation, IEEE Transactions, pages 983 to 993, by J. Anguera et al.
In this article is described a patch antenna, comprising a first radiating element disposed above an earth plane and excited in its fundamental mode by a coaxial probe, and a second radiating element disposed above the first element and excited by the first radiating element by capacitive coupling so that the currents develop in the first radiating element and in their turn excite the second element. Metallic pads allow the connection between the various layers separated from one another by an air layer acting as dielectric so as to electrically insulate the conducting layers from one another.
In this article, the two radiating elements do not have the same size, the second radiating element is larger than the first radiating element. This results in a creation of two separate frequency bands.
The bandwidth of such an antenna is increased with respect to a conventional structure but to the detriment of the size of this antenna which is bulky. It follows from this that antennas of this type are very difficult to integrate since a thickness of the antenna is obtained that is relatively large for the needs of integration into a communicating object.